1. Field of the Invention
The present invention relates to a thermal printing or writing head for printers of data transmission systems, particularly word processing systems.
2. Description of the Prior Art
A thermal writing head generally includes a plurality of resistive elements adapted to have a current flowing therethrough and so to heat up, and to cooperate either directly with a printing medium, a thermosensitive paper, or indirectly with an ordinary printing medium, by means of a ribbon coated with an ink which melts with the heat of the elements. In the first case, direct thermal printing occurs and in the second case a thermal transfer printing occurs.
A series type thermal printing may be employed in which a mobile head has at least one generally vertical strip of elements aligned in an orthogonal direction to the direction of rectilinear travel, during the printing of a line of characters, corresponding to the height of the strip, of a carriage supporting the head.
A line type thermal printing may also be used in which the head, which is immobile, includes a generally horizontal row of numerous elements, of a width compatible with the width of the medium intended to be printed during its travel orthogonally to the row of elements.
From a so called printing card, or a transition card, the resistive printing elements must be supplied with current during given time intervals as a function of the characters, or graph representations, to be printed. For this, the resistive elements are connected electrically to the card in question by conductors adapted for cooperating mechanically and electrically with the terminals of a connector integral with the card.
The most widely used solutions to this connection problem use a specific female connector having a single row of terminals integral with the card.
The resistive elements are formed on a basic substrate, generally a ceramic.
The line printing heads include, as has already been mentioned, a large number of heating elements, therefore formed on a large sized substrate. Heads are often used having 1728 elements. Since ceramic is a costly material, the useful life of these heads is easily that of the printers, all the more so since its elements are relatively little actuated. They do not consequently need to be replaced.
The series printing heads have a much lower number of elements, their basic substrate is much less cumbersome and so much less expensive, and these heads may be interchangeable and interchanged.
By way of example, it is acknowledged that a series head, for printing characters 3 mm high with 8 points per millimeter, having therefore a strip of 24 elements, has a lifespan corresponding only to the printing of 10,000 conventional A4 formats.
A series head with 24 heating elements, for example, has not 48 conductors, nor 25 including one ground conductor, but 30 conductors including 6 ground conductors for taking into account the sections of the conductors and the current densities but this is a high number.
If the pitch of the connection conductors, formed on the substrate of the head, corresponds to the pitch of the terminals of the conductor of the card, the substrate mounted on the support may play directly the row or male connector for the female connector of the card. It will however be readily understood that in the case of a large number of conductors, such as 30, such a solution is not satisfactory for it is based on a large sized and consequently expensive substrate. This solution may even be proscribed in the case of a limited number of conductors, because of the small range of dimensions of female card connectors which limits the reduction of the size of substrates. And the implantation on the substrate of a series-parallel control converter does not for all that overcome the disadvantages.
If the number of conductors or conducting tracks is high, on the one hand, and if the pitch is small by being formed on a substrate as reduced in size as possible, on the other hand, and that remains true with a small number of conductors because of the minimum dimensions of the female card connectors, the solution then consists in interposing a flexible track supporting film with an enlarged connection portion for converting the pitch.
Flexible film series printing heads are known, used as a "carriage", connecting the substrate with its heating elements directly to the electronic cards controlling the current supply. In these embodiments, the flexible film follows a complicated path and the replacement of these heads, with their carriage, can only be carried out by a qualified person and not by the operator of the printer not having the appropriate tools.
Series printing heads are further known having, on a wafer, a heating element substrate with the heating elements and associated conductor portions, a connection support and a double face flexible film, with printed tracks on both its faces, melted or pressed on the substrate at one end for connecting the tracks, on its face turned towards the wafer, to the conducting portions of the substrate, and bonded to the connection support at the other end, where the tracks are printed on its other face so as to be able to cooperate with the terminals of the female card connector.
Providing that the wafer of these heads is rigid, that these heads are compact and so that the length of their flexible film is small, they may be readily dismantled by the operator of the printer.
However, all the connection contacts of these heads, at the end of the flexible film connected to metallized holes, are disposed on the same face: or else the connection is provided by means of specific female connectors, having a single row of terminals, which have been effectively developed for this purpose, as recalled above, or else the connection is made by means of an additional male type connector.
The present invention aims then at providing a series thermal printing head which may be connected to a conventional commercially available female connector, and which may be readily dismantled.